


Growing Pains

by AlfaNumeric



Category: Undertale (Video Game)
Genre: AO3 Stop Merging My Tags Damn You, Adopted Sibling Relationship, Air Inflation, Alternate Universe - Pre-Canon, Asriel is 8, Autistic Chara (Undertale), Belly Expansion, Belly Kink, Body Expansion, Cartoon Physics, Chara is 10, F/M, Hope you guys do too., I had fun with it though., Implied Balloon Kink, Incest, Inflation, Magic Bodies Are Weird, Minor Violence, Paw Play, Slapstick, This is just really weird y'all., Transfem Chara, belly inflation, deflation, helium inflation
Language: English
Status: In-Progress
Published: 2019-06-05
Updated: 2019-06-05
Packaged: 2020-04-08 10:51:35
Rating: Explicit
Warnings: Underage
Chapters: 2
Words: 4,269
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/19105618
Author URL: https://archiveofourown.org/users/AlfaNumeric/pseuds/AlfaNumeric
Summary: Chara ditches her responsibilities, and Asriel has an incident picking up her slack. Cartoonish SFW kink, will get more suggestive in later chapters. Rating X for good measure, but it's clean right now. Chara might get frisky later, we'll see.////////////////////////////////////////////////////////////////////////////OOOOOOOOOOH BOY, I SURE AM ALIVE AREN'T I?? Sorry for disappearing for like, several years, and deleting a couple things. Shit has been REAL complicated, and I'm sibcon/kodocon trash now. As if I couldn't get worse LMAO. Have some Chasriel love, and happy belated Chasriel Day.





	1. Chapter 1

**Author's Note:**

> Pre-game, Chara’s a nonbinary transgirl. Genitals are not specified as they’re not necessary for the purpose of the fic so far. Switching pronouns mid-work seems weird to me but they will use both she/her and they/them in my works with them. <3

        “Hey, I’ll catch up with you in a bit, OK Azzy?”

        Asriel turned his eyes upwards from the box they’d been fixed on. They’d spent the past couple hours organizing their storeroom after Chara’s party, while their parents cleaned up the rest of the house. They’d just gotten done putting away everything but the balloons, and Chara was, expectably, dipping out of finishing up.

        “What, are you gonna leave these helium tanks out here?” said Asriel.

        “They too heavy for you?” replied Chara, answering his question with another. She put her balled-up hands on her hips, tapping her foot teasingly.

        Asriel sighed. “I guess I can handle it, I’ll call if I need anything. It’s your birthday, after all.”

        Chara looked satisfied. “Atta boy. I’m gonna go see if Mom and Dad need any help.”

        And with that, she shut the storeroom door behind her with a _thud_ , leaving Asriel alone with several boxes of party balloons, a helium tank, and the brick room full of shelves and packed boxes. A single fluorescent light buzzed in the background, keeping the place from feeling too desolate.

        Asriel let out a deep sigh, and sat himself down next to a box of balloons, pushing away a helium tank from his intended resting spot. He decided that if Chara were slacking off, he could forgive himself for taking a short break before wrapping things up. He gently leaned back against a shelf, looking up at the dark purple ceiling, his tired arms and legs splayed around him.

        It’d been a year and change now since Chara showed up. It astounded him, really. It simultaneously felt like another life, and like it had just been a day or two ago. The first human to enter the Underground had legitimately surprised him -- she was grim most of the time, but cheery if slightly snotty while interacting with Asriel. He felt as though, if she didn’t find him so fascinating, her respect for him would be drastically reduced.

        At 10 years old (as of today), she was a scant 2 years Asriel’s senior, and yet she spoke and carried herself like an adult. Perhaps that was a human thing, or a girl thing. Would Asriel act like Chara a year or so from now?

_Nah,_ he thought to himself. _I’ll be a lot nicer._

        Not to say Chara was mean-spirited. She frequently hugged and displayed affection for Asriel, like her own brother. She got along very well with the family, and they treated her in kind. Though she was very formal towards them, despite having plenty of time to warm up. Most of the time, she seemed lost in her own head, as if having a conversation with herself that the world would never know about.

        She took interest in things that Asriel would never have considered. Physics, biology (monster and human), mathematics that involved letters, and symbols Asriel didn’t even recognize. But also seemingly more mundane things. She once spent an afternoon examining the tree outside their home, wondering how it had turned such a deep black. Asriel had never put any thought into it, but Chara seemed entranced.

        She’d be found rubbing at her arms, and sometimes with little red marks on her skin as if bitten by an insect. When asked, she would brush it off. “Just a bad habit,” she’d say. They never pressed any further.

_Yeah_ , Asriel thought. _Very fascinating._

        He leaned forward, readjusting his posture. He realized he’d been sitting there quite some time, and he should probably get wrapped up. He turned to the box beside him, and noticed something quite unusual about it.

        There was a bag missing.

        The box was not large, no bigger than Asriel’s handpaws put together, and it held precisely 10 packets of balloons. They’d used five for the party, but there were only four in the box right now.

_Did Chara…_

        Asriel shrugged to himself. Even if she did, it wasn’t like there was much they were using them for anyways. He packed up the box, and picked it up, preparing to put it on the top shelf with the rest.

        The shelf was pretty high up, but Asriel had confidence. He’d put on a solid three inches this year. Chara had kept pace with his growth, and Asriel still only came up to her nose. She’d have had no problem with this reach, but Asriel had to stand on the front of his footpaws to manage it. He slid the box into place, and stepped back down onto his heels…

        ...right on top of the missing packet.

        Asriel slipped, his left heel finding not the brick it was expecting but rather a half-open plastic shred. His footpaw fell out from underneath him, sending the goat boy bottom-first onto the floor, kicking the shelf with his loose foot as he went.

        Asriel caught himself with his hands, and was startled but otherwise unharmed. Winded at worst. But his ears caught an ominous creaking noise as the shelf in front of him began to tip over.

        He gasped, and scrambled to the side to avoid contact with the shelf as it collapsed onto the floor with a mighty crash, as a wave cardboard boxes and a helium tank spilled across the storeroom.

        The child collected himself, finding himself stomach-down on the floor, his vision and hearing both reeling from the crash. He glanced over to the mess that had resulted. From what he could see, boxes had been strewn across the room, and the spare helium tank had fallen onto the floor.

_Oh, jeez, I’m gonna have to pick all this up now_ , Asriel thought, a scowl spreading across his face

        He put his hands on the ground, pushing himself upwards, and it was then that a sharp pain spread throughout his right foot, the one that’d been nearest to the collision.

        “Ow! What the heck?” He turned himself around, and lifted his hurt footpaw, and noticed a hose was sticking out of it. His eyes went wide as his hearing started to return, and he heard a high-pitched hissing noise. As his vision cleared, he began to see that his right footpaw had grown about twice the size of his left, a tingling sensation surrounding its skin. The tank had a needle nozzle applied to it, and it had pierced the heel of his foot, and when the tank landed, it had undone the valve just enough to cause helium to start flowing into Asriel’s body.

        “Oh my stars, what-- what’s happening?” A twinge of worry crept into Asriel’s voice, as he shook his swollen paw to try and get the hose out, to no avail. His magic body had healed the hose into him, despite him not wanting it to. He put his other footpaw on the ground, slowly bringing himself upright.

        He eventually managed to stand, though he had to stand on the front of his puffy foot. It lifted itself off the ground slightly, and Asriel had to push against the ground to make both of his legs level out.

        “Chara!!” Asriel cried. But try as he might, the storeroom door was too thick for anybody to hear him. The pain in his foot had subsided, though it hadn’t stopped growing. His toes were slightly splayed, they too filled with helium. He leaned over and tried to massage it, and noticed that his skin was still soft and compressible, and the helium dispersed around the spot he pushed on, making the environ stretch farther.

        Asriel eventually gripped his footpaw with both hands, squeezing it tightly. As he did so, he felt a sharp jolt through his right leg, as if electrocuted, and his footpaw returned to normal size.

        Asriel found some calm, putting his handpaws on his hips. He then noticed that his sweater wasn’t as loose on him as it used to be. He pulled his sweater up, revealing that his formerly lithe, flat belly had developed a noticeable curve to it, as if he’d eaten too much cake at the party he just finished attending. He felt about as well, the tug of fullness encroaching across his abdomen.

_Of course,_ Asriel said to himself. _The gas had to go somewhere._

        He squished his paws down on his stomach, to see if he could deflate it like he did his foot, but try as he might he wasn’t able to get anything to budge. He was too wide for himself to squeeze into. He turned his attention to the tank, to see if he stop the flow. No such luck, however. The tank’s handle appeared to be bent out of shape. No matter how he twisted or turned, he was unable to close it again, not soon enough. Asriel didn’t want to think about what would happen if he wasted too much time on it, as doing so made him more aware of the growing pressure inside of him.

        He took a deep breath, his diaphragm straining against his swollen form, and lifted his foot to try and pull the hose out again. After a minute or two of tugging, he was finally able to pull the hose free from his foot with a cute _pop!_ He put his foot flat on the floor for the first time since the whole debacle started, and sighed deeply in relief, as difficult as his balloon-belly made that. His stomach had become incredibly bloated, and pushed on his now-tight sweater quite intently, having expanded to about as wide as his torso when he’d started. He didn’t feel any pain, surprisingly, but the hide of his abdomen was pretty tight, as if he’d swallowed a balloon. He patted it with his handpaw, and a hollow tone resonated throughout.

        Balloon. The balloons. Right. That’s what he was here for.

        Asriel wasn’t sure how he was meant to deflate himself, but he’d worry about that in a moment. He straightened himself out, and leaned over his distended tummy to pick up a full packet of balloons from the floor. As he did so, he noticed a second hissing noise as his right heel left the ground. He straightened back up again, and it went away.

        Strange, but okay. Asriel supposed stranger had happened today. He put the packet back in the box, and turned to the fallen shelf. It was a plastic/aluminum shelf, so it wasn’t too heavy. If it _had_ fallen on him, he wouldn’t have been badly hurt. He picked it up, despite his clumsy new shape, and righted it back where it belonged. The secondary hissing noise started again once he leaned over, and once again stopped when he stood.

        Asriel noted that his stomach felt significantly smaller than it did a minute ago, and supposed it was just going away on its own. That was a relief. Guess he wouldn’t have to do any awkward explaining after all. He picked up the damaged tank, and after a while, managed to straighten out the handle on it. He’d have to let Asgore know of the incident, so that the tank could be disposed of safely. The hissing from the tank stopped, but the high-pitched noise continued regardless. Asriel was getting annoyed at this point, and suddenly the obvious occurred to him.

_Wait_ , he thought. _My foot!_

        Asriel lifted his foot off the ground, and as he predicted, he found the needle nozzle from the hose still implanted in his heel. It had become flush with his skin, and there was no way he was going to be able to pull it out. As he examined it, his belly continued to shrink, as if a line were run from his abdomen to the bottom of his foot.

        He left his foot up for a while, and all the helium that had filled his stomach came rushing out of it, leaving his belly nice and flat as it once was.

        “Ahh, much better.” Asriel patted his tum, a solid thump coming from it as he’d expect. The hissing had also stopped, so Asriel came to the conclusion that his troubles were over, save the new hole in his foot. He returned to work, until a knock came through the door.

        “Asriel?? Are you okay?? We heard a crashing noise.” Chara’s voice was muffled, but the concern for her brother’s safety was clear.

        “Yeah! All good here,” Asriel replied. He looked to the mess on the floor. “Had a little accident though, so I might need some help cleaning this mess up.”

        “Ugh. You’re killing me, dude.” Chara opened the door, flipping her hair back as she stomped into the room, stopping dead after about two paces at the sight of the bedlam.

        “Whoa. What happened?” asked Chara.

        “I tried to put a box on the top shelf and slipped on an open packet of balloons,” Asriel said, gesturing to the now-emptied box. He tried to avoid recounting the next several minutes after that.

        Chara looked at the ground and nodded. She pursed her lips as she tried to avoid recounting the reason there was an open packet of balloons on the ground, and why it had taken her so long to react to the incident, though her guilt was visible on her face through her surprise.

        “Hey, Chara?” Asriel asked, his voice flattening out.

        “Yes, Brother?” Chara’s eyes met Asriel’s.

        “Why was another bag of balloons opened? The party ended hours ago.” Asriel held out an open palm to the plastic scrap. Chara’s stood up straight. _Now_ she’d done it.

        “I, uh, needed them for something,” she said.

        Asriel chuffed. He’d gathered as much. “Like what?”

        “I-- um, uh…” Chara began to stammer, trying to avoid showing her whole hand to Asriel. He may have been younger, but he wasn’t born yesterday.

        “You were just playing with them, weren’t you?” Asriel asked, cocking an eyebrow, his expression looking not out of place in a court drama.

        “No!!” Chara yelled. Her face went red, her hands thrown to her sides in fists. Her sudden shout had blown a tuft of fur across Asriel’s eye, which he casually brushed away.

        Chara crossed her arms and avoided eye contact. “I don’t play with toys,” she said, barely a mutter.

        Asriel’s shoulders dropped and he sighed. “You know we don’t mind, Chara,” he said. “You just needed to mention it. I could have been hurt.” He gestured to the mess on the floor.

        Chara stared a hole into a brick on the floor beside her, letting her guard down entirely. Yeah, he got her. Chara Dreemurr’s adult-like facade had been broken by a tiny goat boy who found out that she liked to play with balloons.

        “It was just… embarrassing,” said Chara, scratching the back of her head.

        “What? You don’t like people knowing that you... play?” Asriel seemed puzzled.

        Chara’s voice lowered to just above a whisper. “It’s a long story,” she said, attempting to dismiss the subject.

_What a strange child indeed,_ Asriel thought. Whatever was going on, he wasn’t about to press the issue. No harm, no foul, after all.

        “Well, anyways, can we get this picked up?” Asriel said.

        Chara rubbed her arm like she does, trying to alleviate the embarrassment of the situation.

        “Yeah. Yeah, let’s do that,” she said, before stepping further into the room.

        The two of them picked up the rest of the storeroom undisturbed, and Asriel forgot all about the incident he’d just been through...

        ...until later that evening.


	2. Chapter 2

**Summary for the Chapter:**

> Chara catches wind of Asriel's new body part, and assists in an unforseen side-effect.
> 
> Contains deflation/flattening, mild paw play, and Chara getting real blushy.

        Asriel and Chara had changed into their pajamas, exhausted from the day’s events. The pair walked into their bedroom, a slight tapping accompanying Asriel’s footsteps, that neither of them really noticed.

 

        They crawled into bed, and Asriel closed his eyes. Chara spent a moment looking at him, as she typically did. As her eyes started to close, her brain processed something strange about Asriel.

 

        “Hey, Asriel,” she said, shaking her brother on the shoulder.

        Asriel opened his eyes again. “Huh, wha, what’s up?”

        “Were you always this… short?”

 

        Asriel had no earthly clue what she was talking about. He’d always been shorter than her, but only by a couple inches. He chuckled. “Hey, knock it off. I know you just turned 10, but that doesn’t make you Queen of the Underground all of a sudden.”

        Chara wasn’t laughing. “No, for real. I feel like you’ve shrunk a little bit.”

        Asriel snickered again, a smile across his face. “C’mon, Chara, it’s late.” He rolled over, taking a bit of blanket with him.

        Chara huffed, pulling the goat back around to face her. “I’m serious! You shrank! Hey, what happened?”

        Asriel’s confusion returned. What did she mean, he shrank?

        “What do you mean, I ‘shrank’?” asked Asriel.

 

        “Get up,” Chara said, throwing the rest of the blanket onto Asriel and putting feet on the floor to stand up.  Asriel flinched as he was bombarded with blanket, fighting it off and joining Chara up at the wall of their bedroom.

 

        Toriel had been marking their heights on the doorframe.  Last checked, Chara measured about 5’, Asriel at roughly 4’9”, a solid three inches shorter. But when Asriel stood up against the markers, he found that Chara was right -- he’d lost another three inches!! He was now as tall as he was that time last year!

 

        “What the heck???” Asriel checked himself over. He didn’t notice anything else different, except… except the nozzle.

 

        There was an open nozzle on Asriel’s footpaw. He froze as the situation dawned on him.

 

        “Asriel, what happened in that storeroom?” Chara asked, in response to the expression on Asriel's face.

        Asriel's eyes snapped to meet Chara’s as his skeleton melted back into motion. “Okay. So. Funny story about that.”

 

        He sighed and walked back to the bed, sitting down on its edge. Chara joined him, and he made sure his footpaw was flat on the floor before recounting the whole story. The fall, the crash, the needle embedding itself in his foot, his paw inflating, his belly swelling up, him getting the hose out and deflating himself through the leftover nozzle.

 

        Chara gazed at Asriel’s paw from the moment he mentioned it. “There’s a nozzle in there??”

        Asriel nodded firmly, lifting his paw to show the brass hole inside of it.

        Chara recoiled briefly, before taking a closer look and realizing that there was no blood, no signs of damage… it just looked like part of him now. He wiggled his toes, and the nozzle moved with his foot, as if it were meant to be there.

        Asriel put his foot back on the floor, and Chara looked at his shrunken brother in fascination. “That can just… happen?”

 

        Asriel shrugged. “I suppose so.”

        Chara’s voice emitted equal parts worry and wonder. “But how? How is that possible?? Do we need to tell Mom and Dad?”

        Asriel shook his head vigorously. “Nonono, don’t do that. It’ll be fine. It doesn’t hurt, I promise.”

        “But Asriel, you’ve shrunk!! What if something gets in it? Do monsters get infections?” Chara asked, wonder mostly giving way to worry at this point.

        “No, things like this just happen sometimes. It’ll either go away or it won’t. Not much TO do about it, without risking being hurt more.”

 

        Chara lowered her voice. Right. Monsters were made of magic, not meat. He sustained no damage because the incident was accidental. Something fell on him, he wasn’t attacked. It’d be a far more invasive procedure to have it removed.

 

        “So, what, you’re just gonna deflate out of existence?”

        Asriel shook his head again, more firmly this time. He doubted that. He hardly noticed himself shrink, and supposed he weighed about as much as he did earlier. “I don’t think so. I think I’ll just… go flat. Actually, I don’t know, let’s see.”

 

        Asriel lifted his foot up, and to the pair’s amazement, Asriel began to shrink again. He dropped another inch in height before Asriel noticed any other change in his body -- his form was becoming loose, almost floppy, like a deflated pool float.

 

        Chara started forward. “Asriel!! Are you okay??”

        Asriel was floored, but didn’t seem pained. “Yeah! Yeah, I’m fine.”

 

        Before long, he’d deflated so much that he couldn’t keep his paw on the floor, and he went limp on the bed, his pajamas hanging off of him like a coat hanger.

 

        “Oh, crap,” Asriel remarked, hoping only Chara had heard him. Air continued to rush out of the child, before long rendering him a flat piece of fur on their bed.

 

        “Hey, uh, Chara?” Asriel inquired.

“        Yeah?” Chara replied.

 

        “C--could you help me out here?” Asriel’s voice seemed less nervous, and more bashful. He hadn’t meant to flatten out entirely.

        Chara’s voice shook. “W-what do you want me to do?”

        Asriel sighed, quite the feat for a flattened boy. “What do you think?” He wiggled his toes as best as he could.

 

        Chara feared as much. She had to re-inflate him. She slowly dismounted the bed, taking Asriel’s deflated footpaw in her hand. The metal was still solid, and she was able to pick his paw up by the nozzle.

 

        Suppose she’d just have to put her mouth up to it and blow, just like… one of her balloons. Chara’s heart pounded. This was quite possibly the strangest, most outlandish thing she’d ever done, and yet that wasn’t why she felt awkward.

 

        Chara didn’t just play with her balloons. She, er…  _played_ with them. The child’s sex drive had developed over the past year, and she found gratification in the way they felt in her hands as they swelled up, the way they smelled… It was rather embarrassing, and she now found her mind swirling into places involving Asriel that she didn't think it should. She’d have to get this over with pretty quick, lest she make the situation weird for him too.

 

        She inhaled deeply, before putting her lips firmly against the heel of Asriel’s paw, her face buried firmly in its arch, and blew a firm stream of air into his body with a toon-like _fwoosh_.

 

        Asriel’s body increased, lifting slightly from the bed, and his paw began to shape itself in Chara’s hands. The child blew again, and again, refilling Asriel’s whole body little by little. Her cheeks had gone beet-red, equal parts in embarrassment and effort of re-inflating her little brother.

 

        Soon enough, Asriel was back to his normal self, height and all. Chara put her thumb over the nozzle, trying to avoid it leaking back out again. She breathed heavily, looking around for something she could use to plug the nozzle up. Her eye caught a popped balloon, from her play session earlier. She reached over to grab it, but had to let go of Asriel’s nozzle to reach, allowing the boy to deflate a little bit. Once she grabbed the latex scrap, she muttered out a quick apology before giving the goatpaw one last lungful of air and sealing it shut. A temporary solution, but it would work.

 

        Chara stood up at long last, flopping over onto the bed.

        “Hey Chara?” Asriel said, pinching at his waistline.

        Chara didn’t move, her face buried in her pillow, dead to the world. “Yeah, what’s up.”

        “I think you put in a bit too much. My body feels… tight.”

        Chara turned her head over. “What do you mean?”

        “Like it’s hard to move,” he said.

        “Hard to move?” Chara said.

        “Yeah.”

        Chara reached out, pressing against Asriel’s belly. Indeed, it felt a bit tighter than it should. Usually he had a soft plushness to him. She let her head fall back onto her pillow. “Just exhaust a bit. There’s just some rubber in there until I find something better.”

        “But what if I lose too much air again?”   


_         Jeez, _ Chara thought.  _ He’s talking like he’s turned into some sort of living balloon. _

        Though to be truthful, that wasn’t entirely inaccurate. It was clear that despite his body’s composition not having visibly changed, Asriel’s addition had given him a strange, fascinating new power.

 

        “Fine,” she said, climbing out of bed for the third time that night, and taking Asriel’s paw in her hands. She couldn’t help but notice that it’d expanded just a little bit with the excess pressure, his toes spread out ever so slightly, his arch slightly flatter, his soles slightly sharper. She squeezed it investigatively, feeling the fur between her fingers, the squish in her palm. Her heart rate climbed again, before she caught herself, realizing what she was feeling about her own brother.

 

_Get a grip, Chara_ , she thought, before unplugging Asriel’s nozzle. “Just say when!”

        A few seconds later, Asriel piped up. “That’s good!”

        Chara pressed the rubber back into Asriel’s foot, ensuring she hadn’t done anything else unexpected, before climbing back into bed for what she _hoped_ would be the last time tonight.

        “Thanks, Chara,” Asriel said.

        “Don’t mention it,” Chara groaned in reply.

 

_         No. Seriously. Don’t mention this _ , she added to herself, before the both of them nodded off.

 

        Chara spent the night dreaming about what she’d seen. She fantasized about what Asriel looked like with his belly overfilled, her brain involuntarily drawing connections between that image and the balloon she’d enjoyed herself on, now stuck inside of Asriel. She allowed herself to entertain these fantasies in the safety of her mind, picturing her brother’s stomach swelling up like a blimp, becoming tight and impregnated, her hands buried in his fur… She’d have to take advantage of this at some point soon. Surely, there were justifiable reasons, right?

 

        She didn’t just start crushing on her brother because of her balloon fetish…

        ...right?

**Notes for the Chapter:**

> Spoiler alert: yes, she did. We'll get into that next time on Dragon Ba--I mean on Whatever The Fuck This Nonsense Is Turning Into!!


End file.
